The first book was awesome, the second okay, the third was a train wreck. So much potential in the first book and then he goes and ends the series with "The Amber Spyglass". It wasn't even so much the ending of the last book that was awful. He decided not to go with the easy, happy ending; I can respect that. It was everything that came before that. It comes across like he got to the final book in the series and realized that he actually needed to wrap-up everything that came before.
And don't get me started on Will. At the beginning of "The Subtle Knife" he was sympathetic, halfway through though he was annoying. By the end of the series however, he just needed to go away. That's probably why I was okay with the series ending. As soon as Will entered the picture everybody just became a caricature of themselves. Lyra from book one to book two seems to have undergone a personality transplant. I understand she went through something deeply traumatic in the previous book and it would be unrealistic to not show any of the effects of that, but it only seems to show when Will's present. It's like he sucks the true personality out of anybody he's in the same room with for more then ten seconds. Not every character in a book should be written to be liked. In fact I have read books where none of the characters were likeable, but characters that were previously if not likeable at least understandable, acted like they had encountered some mysterious force that distorted their personality. And the forces name was Will.